As this sort of exhaust gas sampling apparatus, as disclosed in JP-A2001-264223, there is one that connects a diluent gas flow path for introducing diluent gas to a main flow path through which exhaust gas introduced from an exhaust gas introduction port flows, and on a downstream side of a connecting points between the paths, samples part of diluted exhaust gas to collect the part in a diluted exhaust gas collecting bag. Also, the exhaust gas sampling apparatus samples part of the diluent gas from the diluent gas flow path to collect the part in a diluent gas collecting bag in order to make a background correction of the concentration of a component in the diluted exhaust gas collected in the diluted exhaust gas collecting bag. Further, the exhaust gas sampling apparatus calculates the concentration of the component in the exhaust gas by subtracting the concentration of the component in the diluent gas in the diluent gas collecting bag from the concentration of the component in the diluted exhaust gas in the diluted exhaust gas collecting bag.
Specifically, this exhaust gas sampling apparatus is configured to provide the main flow path with a sampling venturi to collect the diluted exhaust gas in the diluted exhaust gas collecting bag at a constant flow rate, and provide the diluent gas flow path with a needle valve to collect the diluent gas in the diluent gas collecting bag at a constant flow rate. Further, in order to, within a sampling time for the collection into the diluted exhaust gas collecting bag, make an amount of the diluted exhaust gas collected in the diluted exhaust gas collecting bag and an amount of the diluent gas collected in the diluent gas collecting bag substantially the same, the exhaust gas sampling apparatus is configured to make a critical flow rate of the sampling venturi and a control flow rate of the needle valve substantially the same.
For example, in order to change the sampling flow rate or the like along with a change of a test cycle, or perform other operation, it is considered to provide a plurality of sampling venturis to switch or combine among the sampling venturis. In this case, in order to, within the sampling time, make the diluted exhaust gas amount collected in the diluted exhaust gas collecting bag and the diluent gas amount collected in the diluent gas collecting bag substantially the same, it is possible to parallel provide a plurality of needle valves each having the same control flow rate as a critical flow rate of each of the plurality of sampling venturis. Further, in the case of combining among the plurality of sampling venturis to control a diluted exhaust gas flow rate, it is necessary to combine among the plurality of needle valves to control a diluent gas flow rate.
However, although a needle valve determines a flow rate on the basis of a differential pressure between pressures on upstream and downstream sides of the valve, in the case of combining among the plurality of needle valves for use, a pressure condition is different from that of a single body of the valve, and fluid does not flow at a set flow rate, so that the same flow rate as the diluted exhaust gas flow rate cannot be obtained. For this reason, within the sampling time, the diluted exhaust gas amount collected in the diluted exhaust gas collecting bag and the diluent gas amount collected in the diluent gas collecting bag are different from each other, which give rise to a problem that it is difficult to accurately make the background correction.
Also, it is possible to, for each of a plurality of diluted exhaust flow rates obtained by combining among the plurality of sampling venturis, provide a plurality of needle valves each having the same control flow rate as the diluted exhaust gas flow rate; however, there is a problem that an apparatus is increased in size to increase cost.